The present invention relates to aqueous coating compositions for the manufacture of cast coated paper and, more particularly, to coating compositions for cast coated paper having a high gloss, and to a method of producing cast coated papers at improved operating speeds, as well as to cast coated papers produced with said coating compositions.
Conventional methods of producing high gloss coated papers for printing, known as cast coated papers, include (1) the wet casting method adapted to produce a glossy coated paper by pressing the paper carrying a layer of wet coating against the surface of a heated drum having a highly polished finishing surface, (2) a gel-casting method adapted to produce a high gloss coated paper by pressing the paper carrying a layer of coating in the gel state against a heated polished drum, and (3) a rewet casting method which which employs a paper that has been coated and dried, following which the surface of the coated layer is then plasticized with a re-wetting agent and is pressed against the surface of a heated polished drum.
In each of these conventional casting methods, the surface of a coated layer in a plasticized state is pressed against a heated finishing surface (e.g., a heated, polished chromium plated drum) and dried in contact therewith and then released from the drum as a replica of the polished surface. Since the surface of the coated layer is dried in contact with the heated finishing surface in so producing cast coated papers, the moisture in the coated layer or the moisture of the re-wetting liquid must all pass through the paper layer to the opposide side and be evaporated. Because of the manner in which cast coated paper is thus dried, the speed of operation is extremely low at present, as compared to the rate of production of art papers and other similar coated papers, from which the moisture is evaporated directly from the surfaces of the coated paper. In addition, in the production of conventional cast coated papers a primary concern is to obtain a high gloss surface. Accordingly, a plate-like, crystalline kaolin which readily develops a high gloss has been the primary pigment used in formulating the coating compositions. Kaolin is oriented laterally not only on the surface of the coated layer but also within the coated layer in the same manner. Consequently, the conventional cast coated paper, while having a high gloss, exhibits relatively low moisture permeability. Thus, an important technological objective in the cast coated paper field has been to improve productivity by increasing the manufacturing speed.
In order to enhance the speed of manufacture of cast coated paper, consideration has been given to the pigment content of the coating. For example, it is known that pigments such as aluminum hydroxide, titanium dioxide, barium sulfate, precipitated calcium carbonate, natural ground calcium carbonate, calcium sulfite or zinc oxide, which have good moisture permeability as compared to kaolin, can be used instead of the kaolin or together with the kaolin.
If such pigments are used the moisture permeability is effectively improved as compared to the case in which only kaolin is used, but the gloss of the cast coated paper will be decreased and the ink gloss of the printed paper also will be reduced.
When calcium carbonate, for example, is used as a pigment for the cast coated paper, it is considered that a decrease in the gloss is caused by the anisotropic shape of particles of precipitated or natural ground calcium carbonate, such as oblong, spindly, spherical or amorphous shape but not cubic shape.